SUV or Hybrid? A Familial, MPG Conundrum

RAY: This puzzler came to us from Rabbi Jeff Glickman. Here it is:
You drive a gas-guzzling SUV that gets a whopping 10 miles per gallon. Your sleek and efficient spouse drives a sleek and efficient hybrid that gets 100 miles per gallon.

Now, let's assume that both of you drive the same distance each year. Your spouse sees an ad for a new, super-duper hybrid that gets 200 miles per gallon. She is lobbying to trade in her old, wasteful 100-mile per gallon hybrid for the new 200-mpg model; her thinking being that getting this new hybrid to replace the old one will really improve the average miles per gallon of your household.

On the horns of a dilemma, you seek out the one person you trust with questions automotive: your mechanic, Crusty!

"What can I do," you ask, "to improve our household's miles per gallon without buying this new hybrid?"

Crusty says, "Well, if we tune up your old SUV, and inflate the tires correctly, and remove that four inches of accumulated bird poop on the roof, I'm pretty sure we can get you all the way up to 11 miles per gallon." Your heart sinks.

What should you do? That's the question. Under which scenario would your household see the biggest improvement in miles per gallon: by getting the new hybrid, or by tuning up the old SUV?